Polyaxial bone screw

Abstract

A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a head, a retaining ring and a nut. The external capture structure surface includes a threaded portion and a smooth portion; the smooth portion for slidably mating with the retaining ring and the threaded portion for rotatable attachment to the nut within a cavity of the head. The nut fixes the retaining ring to the shank and also provides a tool engagement formation for driving the shank body into bone.

1. A polyaxial bone screw assembly comprising:
 (a) a shank having a body for fixation to a bone and a capture structure extending from the body, the capture structure having an outer surface with a first helically wound guide and advancement structure and a smooth surface disposed between the first guide and advancement structure and the body; 
 (b) a head having a top portion and a base, the head top portion defining an open channel adapted to receive a rod implant, the base having a seating surface partially defining a cavity, the channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening sized and shaped to receive the capture structure therethrough; and 
 (c) a retaining ring having an external surface and an internal surface defining a central bore configured to slidably mate with the smooth surface of the capture structure, the retaining ring external surface configured to be in slidable mating engagement with the seating surface of the head so as to enable selective angular positioning of the shank body with respect to the head; and 
 (d) a fastener having an internal surface with a second helically wound guide and advancement structure thereon, the first helically wound guide and advancement structure configured to rotatably mate with the second helically wound guide and advancement structure to secure the fastener and the retaining ring to the capture structure within the head cavity, the fastener having structure thereon adapted to mate with a tool for operably implanting the shank in a bone of a patient, when the shank, retainer ring, fastener and head are assembled, the shank capture structure extending into the channel so as to engage a rod placed in the channel under pressure and thereby lock the shank in a desired angular configuration with respect to the head. 
 
     
     
       2. The assembly of  claim 1  wherein the fastener is a nut having a faceted outer surface with at least one facet of the nut having a weakened area compressible by an outside pressing force directed at the weakened area, and when attached to the capture structure at least one of the first and second guide and advancement structures being deformable by the outside force pressing on the weakened area, interlocking the first and second guide and advancement structures. 
     
     
       3. The assembly of  claim 1  wherein the shank capture structure includes an annular seating surface and the retaining ring has a top and a bottom, the retaining ring bottom abutting the annular seating surface when the retaining ring is received on the capture structure and the fastener is rotated onto the capture structure with the first and second guide and advancement structures mated and the fastener pressing against the retaining ring top. 
     
     
       4. The assembly of  claim 1  wherein the head cavity is partially defined by an upper shoulder portion having a planar surface disposed perpendicular to an axis of rotation of the head. 
     
     
       5. The assembly of  claim 4  wherein the upper shoulder portion includes first and second coplanar surfaces. 
     
     
       6. The assembly of  claim 1  wherein:
 (a) the head seating surface that is located to be in slidable mating engagement with the retaining ring external surface is substantially spherical; and 
 (b) the retaining ring external surface that is in slidable mating engagement with the head seating surface is substantially spherical. 
 
     
     
       7. The assembly of  claim 1  wherein the retaining ring has a planar top surface adjacent the fastener and the fastener is a nut having a faceted outer surface, the planar top surface and faceted outer surface cooperating to provide non-slip engagement by a tool for driving the shank body into bone. 
     
     
       8. The assembly of  claim 1  wherein the first helically wound guide and advancement structure is a raised helical rib. 
     
     
       9. The assembly of  claim 1  wherein the second helically wound guide and advancement structure is a raised helical rib. 
     
     
       10. The assembly of  claim 1  wherein the retaining ring is sized and shaped to be loadable into the head through the open channel and the shank is sized and shape to be loadable into the head through the base opening. 
     
     
       11. The assembly of  claim 1  further comprising a closure structure insertable into the head, the closure structure for operably urging the shank in a direction to frictionally lock the position of the retaining ring external surface relative to the head seating surface, thereby locking the shank body in a selected angle with respect to the head. 
     
     
       12. The assembly of  claim 11  wherein:
 (a) the head has upstanding spaced arms defining the open channel, the arms having guide and advancement structures on an inside surface thereof; and 
 (b) the closure structure is sized and shaped to be positionable between the arms for closing the channel, the closure structure having a closure guide and advancement structure for rotatably mating with the guide and advancement structures on the arms, biasing the closure structure upon advancement rotation against a rod disposed in the channel. 
 
     
     
       13. The assembly of  claim 11  wherein the capture structure end has a dome sized and shaped to extend into the channel for engagement with a rod when received in the head and wherein the closure structure is adapted to operably urge the rod against the dome upon the closure structure being positioned in the head. 
     
     
       14. In a polyaxial bone screw assembly for surgical implantation and including a shank having an upper end and a threaded body for inserting into a bone and a head having an outward opening channel adapted to receive a rod within the channel, the head having a shank receiving opening, the improvement comprising:
 (a) a capture structure disposed on the shank upper end sized and configured to be uploaded through the shank receiving opening, the capture structure having an outer surface with a first helically wound guide and advancement structure and a smooth cylindrical surface disposed adjacent the first helically wound advancement structure; 
 (b) a retaining ring having an external surface and a smooth inner cylindrical surface defining a bore, the smooth inner cylindrical surface configured to slidably mate with the smooth cylindrical surface of the capture structure; and 
 (c) a nut having a faceted external surface and an internal surface with a second helically wound guide and advancement structure, after assembly the nut being positioned between the channel and the retaining ring and operably receiving a tool to drive the shank into a patient's bone, the first helically wound guide and advancement structure configured to rotatably mate with the second helically wound guide and advancement structure to secure the retaining ring to the capture structure within the head, when assembled and in a non-locked configuration, the retaining ring external surface enabling selective angular positioning of the shank with respect to the head, the shank capture structure extending into the channel after assembly to engage the rod and, under pressure from the rod, the retaining ring is urged against the head and locked in a selected angular configuration with respect to the head. 
 
     
     
       15. The improvement of  claim 14  wherein:
 (a) the head has an inner substantially spherical seating surface partially defining a cavity, the cavity communicating with both the channel and the shank receiving opening; and 
 (b) the retaining ring external surface is substantially spherical and in slidable mating engagement with the head seating surface. 
 
     
     
       16. The improvement of  claim 14  wherein at least one facet of the nut has a weakened area compressible by an outside pressing force directed at the weakened area, and when attached to the capture structure at least one of the first and second guide and advancement structures being deformable by the outside force pressing on the weakened area, interlocking the first and second guide and advancement structures. 
     
     
       17. The improvement of  claim 14  wherein the shank capture structure includes an annular seating surface and the retaining ring has a top and a bottom, the retaining ring bottom abutting the annular seating surface when the retaining ring is received on the capture structure and the nut is rotated onto the capture structure with the first and second guide and advancement structures mated and the nut pressing against the retaining ring top. 
     
     
       18. The improvement of  claim 14  wherein the first helically wound guide and advancement structure is a raised helical rib. 
     
     
       19. The improvement of  claim 14  wherein the second helically wound guide and advancement structure is a raised helical rib. 
     
     
       20. A polyaxial bone screw assembly comprising:
 (a) a shank having a body for fixation to a bone and a capture structure extending from the body, the capture structure having an annular seating surface and a cylindrical surface with a first helically wound guide and advancement structure and a smooth surface disposed between the first guide and advancement structure and the annular seating surface; 
 (b) a head having a top portion and a base, the head top portion defining an open channel for receiving a rod, the base having a substantially hemispherical seating surface partially defining a cavity, the channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening sized and shaped to receive the capture structure therethrough; 
 (c) a retaining ring having a substantially hemispherical external surface configured to slidingly mate with the substantially hemispherical seating surface of the head, and an internal surface defining a central bore configured to slidably mate with the smooth surface of the capture structure, the retaining ring external surface configured to be in slidable mating engagement with the seating surface of the head so as to enable selective angular positioning of the shank body with respect to the head; and 
 (d) a nut that after assembly is located between the retaining ring and the channel and having a faceted outer surface and an internal surface having a second helically wound guide and advancement structure thereon, the first helically wound guide and advancement structure configured to rotatably mate with the second helically wound guide and advancement structure to frictionally fix the retaining ring between the nut and the annular seating surface of the capture structure, at least one facet of the nut having a weakened area compressible by an outside pressing force directed at the weakened area, at least one of the first and second guide and advancement structures being deformable by the outside force, thereby interlocking the first and second guide and advancement structures and locking the retaining ring between the nut and the annular seating surface within the head, the nut being sized, shaped and positioned to receive a tool for driving the shank into a bone of a patient and removing the shank from the bone, the shank capture structure extending into the channel after assembly and receiving pressure from a rod in the channel so as to bias the shank and retaining ring against the head and thereby lock the angular configuration of the shank relative to the head. 
 
     
     
       21. The assembly of  claim 20  wherein the head cavity is partially defined by an upper shoulder portion having a planar surface disposed perpendicular to an axis of rotation of the head. 
     
     
       22. The assembly of  claim 20  wherein the upper shoulder portion includes first and second coplanar surfaces. 
     
     
       23. The assembly of  claim 20  wherein the retaining ring has a planar top surface adjacent the nut faceted outer surface, the planar top surface and faceted outer surface cooperating to provide non-slip engagement by a tool for driving the shank body into bone. 
     
     
       24. The assembly of  claim 20  further comprising a closure structure insertable into the head, the closure structure for operably urging the shank in a direction to frictionally lock the position of the retaining ring external surface relative to the head seating surface, thereby locking the shank body in a selected angle with respect to the head. 
     
     
       25. The assembly of  claim 24  wherein:
 (a) the head has upstanding spaced arms defining the open channel, the arms having guide and advancement structures on an inside surface thereof; and 
 (b) the closure structure is sized and shaped to be positionable between the arms for closing the channel, the closure structure having a closure guide and advancement structure for rotatably mating with the guide and advancement structures on the arms, biasing the closure structure upon advancement rotation against a rod disposed in the channel. 
 
     
     
       26. The assembly of  claim 24  wherein the capture structure end has a dome sized and shaped to extend into the channel for engagement with a rod when received in the head and wherein the closure structure is adapted to operably urge the rod against the dome upon the closure structure being positioned in the head. 
     
     
       27. A polyaxial bone screw assembly method comprising:
 (a) inserting a retaining ring through a channel and into a cavity of a head, the head channel opening outwardly and adapted to receive a rod within the channel, the head cavity being disposed between and communicating with both the channel and a shank receiving opening, the retaining ring having an inner cylindrical surface defining a bore; 
 (b) inserting a capture structure of a bone screw shank through the shank receiving opening of the head and into the cavity thereof, the capture structure being integral with an elongate threaded shank body and having an outer surface with a first helically wound guide and advancement structure thereon adjacent to a smooth cylindrical surface, the shank, after assembly with the head, extending into the channel; 
 (c) attaching the capture structure to the retaining ring within the cavity by inserting the capture structure through the bore until the inner cylindrical surface of the retaining ring is in sliding engagement with the smooth cylindrical surface of the capture structure, and inserting a nut having a second helically wound guide and advancement structure onto the capture structure and rotating the nut, mating the first and second helically wound guide and advancement structures; 
 (d) subsequent to step (c) utilizing a tool applied to the nut to drive the shank into a bone, and 
 (e) after the shank is driven into a bone, placing a rod in the channel and applying pressure to the rod to engage the shank and lock the shank in a fixed angular configuration with respect to the head. 
 
     
     
       28. The method of  claim 27  further comprising:
 (d) driving the shank body into bone by rotating the shank body with a tool engaged with the nut. 
 
     
     
       29. The method of  claim 28  further comprising:
 (e) subsequently inserting a rod into the channel; and 
 (f) biasing the rod against the capture structure by inserting a closure structure into the channel. 
 
     
     
       30. A method of assembling a polyaxial bone screw comprising the steps of:
 (a) providing a bone screw shank, head, retaining ring and nut; 
 (b) providing the shank with an upper threaded capture structure; 
 (c) providing the retaining ring with an inner cylindrical surface defining a bore; 
 (d) providing the head with a central cavity, a rod receiving channel and a shank receiving bore connecting the cavity with an underside of the head; 
 (e) loading the retaining ring into the cavity; 
 (f) uploading the shank capture structure into the cavity through the shank receiving bore and the retaining ring bore; 
 (g) screwing the nut onto the shank capture structure while within the head; 
 (h) driving the shank into a bone by utilizing a tool placed on the nut; 
 (i) extending the shank into the channel; and 
 (j) placing a rod in the channel to engage the shank so as to bias the shank and retainer against the head and lock the shank in a fixed angular configuration with respect to the head. 
 
     
     
       31. The method according to  claim 30  including the steps of:
 (a) providing the head with an upwardly open channel that communicates with the cavity; and 
 (b) downloading the retaining ring and the nut through the channel into the cavity.